Sand molds are commonly used as cores for casting processes in which a flowable material is cast around the sand core to form a part. When sand is employed to form the sand cores, it must be conditioned and controlled to give satisfactory and uniform results. Typically, the sand is conditioned with additives to meet four requirements: refractoriness, cohesiveness, permeability, and collapsibility. The various methods for conditioning sand generally fall within two broad categories: the "hot box" method and the "cold box" method. Each method requires the additives to be combined with the sand and then the mixture to be cured. The curing stage is what differentiates the two methods.
U.S. Pat. No. 4,050,500 issued Sept. 27, 1977, titled "Method of Making a Shell Mold" discloses one example of the hot box method in which a mixture of sand and a thermosetting binder are injected into a mold. After injection, the mold is heated to cure the composition. Examples of the hot box method are disclosed in U.S. Pat. No. 3,461,948 issued Aug. 19, 1969, titled "Blow Plate Assembly", and U.S. Pat. No. 4,068,701 issued Jan. 17, 1978, titled "Refractory Materials".
There are several disadvantages in the hot box method. Costly heating equipment is needed to heat the core box and keep it hot to polymerize the resin. If the core box cools, the lag time necessary to reheat the core box slows production time. Also, the temperature of the core box must be closely regulated to insure proper polymerization of the resin. Further, the core box is costly because it must be able to withstand the continuous heat.
The cold box method does not require the addition of heat. Typically, the cold box method requires a resin to be mixed with the sand and then polymerized by the action of a curing agent such as catalyst. Examples of resin and catalyst mixtures are disclosed in the following U.S. Patents:
______________________________________ U.S. Pat. No. Issue Date Title ______________________________________ 4,540,724 9/10/85 Phenolic Resin- Polyioscyanate Binder Systems Containing a Phosphorus Halide and Use Thereof 4,421,873 12/20/83 Oxidatively Coupled Cold-Set Binders 4,366,266 12/28/82 Binder Compositions and Process for Making Molded Products Therewith 4,268,425 5/19/81 Phenolic Resin- Polyisocyanate Binder Systems Containing a Drawing Oil and Use Thereof 4,176,114 11/27/79 Process for Manufacturing Sand Cores and Molds 4,070,196 1/24/78 Binder Compositions 3,947,420 3/30/76 Method for Producing Foundry Molds and Cores as Well as Products Thereby Obtained 3,590,902 7/6/71 Production of Foundry Cores and Molds ______________________________________
Typically, in the cold box method, a mixture of sand and resin is blown into the core box in one station of the forming operation. The core box is then transferred by a transfer mechanism to a station for adding the catalyst or alternately, the station is transferred to the core box. A catalyst is then introduced into the mold causing the resin to harden.
One of the disadvantages encountered in the cold box method is the necessity of transferring the core box. Depending upon the size and complexity of the machine and core box, the cost of the transfer mechanism could account for several thousand dollars. Further, the machine cycle time is increased by 3 to 10 seconds for the transfer motion and the core requires between 1 and 10 seconds for the addition of the catalyst, excluding purge time. Still further, there is the problem of premature curing of the catalyst/mixture composition in the cold box methods that do not employ an additive station.
One attempt was made to solve the problem of premature curing in one type of the cold box method. Typically, in this type of method, the catalyst and resin coated sand are mixed before they are introduced into the forming box. When this method is used, the curing process begins before the catalyzed-resin sand mixture is actually forced into the forming box. Less reactive resin-catalyst mixtures are not completely satisfactory because they require longer time to cure which slows production. U.S. Pat. No. 3,994,332 issued Nov. 30, 1976 and titled "Apparatus and Method for Manufacturing Cores and Molds With Means for Independently Releasing Catalyst and Resin Mixes", discloses a method and apparatus for forming core molds having two hoppers, wherein the first hopper dispenses a catalyst-polymerizable resin coated sand into a mixing tube while the second hopper dispenses a catalyst coated sand into the mixing tube. The two coated sand particles intermix prior to being injected into the mold resulting in minimal polymerization prior to injection.
The main disadvantage with the method disclosed in U.S. Pat. No. 3,994,332 is the complexity of the machinery required to mix the two sands. The machine is clearly not adaptable to present forming machinery and would require total replacement of all machinery in the foundry.